*open-rename-prefix*

This variable and the variables
*open-rename-suffix*
and *open-rename-function*
work together in renaming a file opened with open
with the direction keyword argument either
:output or :io and
the if-exists keyword argument either
:rename or
:rename-and-delete. The function that is the initial value of *open-rename-function*
renames the existing file by prepending the value of this variable,
moving the current file type into the filename, and making the value
of *open-rename-suffix*
the new type.

The initial value of this variable is the empty string
"" and that of
*open-rename-suffix* is the
string "bak". If all three variables have their initial
values, the file foo.txt will be renamed foo.txt.bak
(with filename foo.txt and file type bak).